Semiconducting diamonds are drawing attention as new materials for use in semiconductor devices, including diodes, transistors and sensors. Diamonds are known widely today as good electrical insulators. However, the diamonds of interest in the present invention are semiconducting diamonds with low specific resistance. Diamond has a wide energy gap (5.5 eV) and a high carrier mobility (2,000 cm.sup.2 /V.multidot.sec) and is both thermally and chemically stable. Thus, semiconducting diamonds hold significant promise as excellent materials for use in environmentally resistant, high-speed or power devices and blue light emitting devices.
Semiconducting diamonds are available in three/types, natural bulk, high-pressure synthesized buld and vapor-phase synthesized thin film. Diamonds of p-type are obtained by doping the diamond film with boron (B). Diamonds of n-type are obtained by doping the diamond film with phosphorus (P) or lithium (Li). The n-type diamonds possess high resistance. Low-resistance n-type diamonds are not yet to be obtained.
Bipolar devices cannot be fabricated from semiconducting diamonds. However, Schottky diodes which make use of Schottky junction between tungsten (W) and a p-type diamond, as well as several types of unipolar transistors are currently being fabricated in the laboratory.
Methods which are commonly used to grow thin films of semiconducting diamonds include microwave plasma assisted CVD techniques and a hot filament assisted CVD techniques. In these methods, feed gases such as CH.sub.4, CO and H.sub.2 are decomposed using a microwave induced plasma or a hot filament. This results in the formation of thin diamond films on a heated substrate made of silicon (Si), molybdenum (Mo), diamond, etc. Addition of B.sub.2 H.sub.6 to the feed gases allows thin films of p-type diamond can be obtained.
Cubic boron nitride (c-BN) has also shown promise as a wide bandgap semiconductor. Specifically, c-BN is drawing attention as a material suitable for use in environmentally resistant devices, power devices and ultraviolet to blue light emitting devices. Like diamond, c-BN is also a material that has high chemical, thermal and physical stability. In addition, c-BN has a wide bandgap.
Boron nitride (BN) is a compound made up of boron and nitrogen and is available in various structures including h-BN, t-BN and a-BN. Hexagonal boron nitride (h-BN) is easy to prepare. Cubic boron nitride systems (c-BN) are not easy to prepare. Doping with Be, produces p-type c-BN. Doping with S or Si produces, n-type c-BN.
In order to fabricate devices which use semiconducting diamonds or c-BN, it is necessary to provide thin films of high-quality crystals with few dislocation and point defects. Conventionally, these films are formed by heating the substrate to a suitable temperature under a suitable degree of vacuum and introducing the feed gases immediately to grow a thin film.
FIG. 2 illustrates the conventional process of forming thin diamond or c-BN films. The horizontal axis of the diagram in FIG. 2 plots time and the vertical axis plots the temperature of the substrate. Above the temperature profile are indicated the steps of the process, temperature elevation, growing the film, and temperature lowering. Two successive steps are differentiated by a vertical line. The gas or gases to be introduced are indicated in the space defined by the temperature line and the partition lines.
After a suitable vacuum is drawn, H.sub.2 gas is introduced into the vacuum chamber and the temperature is raised. When the substrate has been heated to an appropriate growth temperature, feed gases, CH.sub.4, H.sub.2 and optionally, B.sub.2 H.sub.4, are introduced. B.sub.2 M.sub.6 is added to the reaction when a p-type crystal is to be formed.
The introduced gases are excited by microwaves, heat, a high-frequency plasma, and other excitation methods. This causes the feed gas to enter into a vapor-phase reaction. This reaction forms a thin film of diamond or c-BN on the substrate.
However, the thin films obtained by the use of these conventional methods of vapor-phase synthesis contain a fairly large number of detects. In thin films, those which are not thicker than 1 .mu.m, the deterioration in the quality of the film is substantial. These low quality thin films make it impossible to fabricate devices that make use of the inherently good physical properties of the starting materials. As will be discussed hereinafter, this results because either impurities remain or oxide films form on the surface of the substrate. An object, therefore, of the present invention is to provide a method of vapor-phase synthesis of a thin diamond or c-BN film that has sufficiently high quality to justify their use in semiconductor and other suitable devices.